1. Field of the Invention
The present invention relates to wireless digital communications.
2. Description of Related Art
A block diagram of a typical 802.11a/g transmitter is shown in FIG. 1. Such a transmitter is a Single-Input-Single-Output (SISO) system. Bits to be transmitted are applied to a forward error correction (FEC) encoder 101, followed by a interleaver 103. Output bits of the interleaver 103 are grouped and mapped within the signal plane by a symbol mapper 105 (e.g., a QAM mapper) to form symbols. An IFFT operation 107 then follows in which symbols are mapped to a series of subcarrier frequencies (i.e., frequency bins) and transformed to obtain a series of time samples. A cyclic extension operation 107 (equivalent to adding guard symbols) is performed to obtain a resulting OFDM symbol. Pulse shaping 109 and IQ modulation 111 are then performed to obtain an RF output signal 113.
A typical 802.11a/g system has a block interleaver (e.g., block interleaver 103) that may be described in terms of a first permutation followed by a second permutation using the following parameters:
N_CBPS is the size of the interleaver, i.e., the number of coded bits per symbol
k is the index of the input bits
i is the index after the first permutation
j is the index after the second permutation
The first and second permutations are as follows:i=(N—CBPS/16)(k mod 16)+floor(k/16), k=0, 1, . . . , N—CBPS−1  1st permutation
there are 16 columns and N_CBPS/16 rows
bits are written row by row and are read column by columnj=s*floor(i/s)+(i+N—CBPS−floor(16*i/N—CBPS))mod s, i=0, 1, . . . , N—CBPS−1  2nd permutation
where s=max(N_BPSC/2, 1), N_CBPS is the number of bits per symbol in the OFDM subcarrier. For different columns, the bit significance index is changed so that the adjacent bits are not always mapped to the same index in any symbol.
The foregoing permutations are represented by blocks 201 and 203 in FIG. 2
Following the strong market success of 802.11a/b/g wireless networking, an 802.11n working group was formed in 2003, chartered to create a standard for high-throughput wireless LAN. In this proposed standard, the maximum data rate can go as high as 720 Mbps with more than twice the range as compared to 802.11a/b/g. The fundamental technology is called Multiple-Input-Multiple-Output (MIMO), which essentially uses multiple antennas to exploit path diversity in the wireless medium. When discussing a MIMO system, M×N means M transmit antennas and N receive antennas.
Multiple antennas makes possible a type of coding referred to as Space Time Block Coding (STBC), an example of which is Alamouti coding. In STBC, a block of information is encoded and transmitted over multiple antennas (space) and over multiple symbol periods (time).
It is desirable that 802.11n (MIMO) systems be backwardly compatible with at least 802.11a/g (SISO) systems. With respect to interleaving in particular, a need exists for interleaving arrangements that achieve backward compatibility while addressing competing design objectives (e.g., compactness, low power consumption, and robustness of communications).